1. Field of the Invention
The present invention relates to a method for the preparation of heterogeneous silicone rubber moldings. More specifically, the present invention relates to an efficient method for the preparation of silicone rubber moldings in which different types of silicone rubbers have been molded into a single body or mass.
2. Background Information
The contact element of, for example, the keyboard for electronic equipment such as a desk calculator or computer, typically has a structure in which different types of silicone rubbers, e.g., an insulating rubber and an electrically conductive rubber, are molded into a single body or element.
The following are exemplary of the methods which have been used heretofore for the preparation of such heterogeneous silicone rubbers moldings:
(1) the two types of silicone rubbers (insulating and conductive) are first molded separately, and the resulting separate insulating rubber component and conductive rubber component are then joined into a single body through the use of an adhesive; or PA1 (2) the unvulcanized conductive silicone rubber composition is introduced into the mold, the already vulcanized insulating rubber component molding is subsequently inserted, and press molding is then carried out. PA1 (1) the silicone rubber composition has undergone crosslinking with partial formation of a three-dimensional network structure to give a cure condition in which it no longer exhibits fluidity under normal conditions of quiescence but does exhibit deformation or limited fluidity under the application of stress; PA1 (2) the organopolysiloxane in the semicured material in such a semicured condition retains residual crosslinking functional groups, and a rubbery elastic silicone rubber can be obtained by the completion of the cure through their further reaction; and PA1 (3) said semicured product has a hardness of zero as measured using the spring-type hardness tester type A (JIS A hardness meter) specified in JIS K 6301, "Physical Testing Methods for Vulcanized Rubber" (similar to ASTM D 2240), and/or a hardness not exceeding 50.degree. as measured by an Asker C hardness meter (spring-type hardness tester from Kobunshi Keiki Kabushiki Kaisha).
However, these methods are encumbered by various problems in the case of heterogeneous silicone rubber moldings in which one of the molded rubber elements is very small or has a complicated shape and/or placement, for example, as in the aforementioned electronic devices. Thus, in the case of the former method, the gluing operation is very tedious and time-consuming when a very small conductive rubber element must be precisely placed in a complex configuration. As a result, in such situations this method suffers from a poor productivity and a reduced dimensional accuracy. Moreover, since the adhesive strength between the different types of silicone rubber components is extremely variable, this method also suffers from a high product defect rate. The latter method is characterized by relatively good dimensional accuracy, but the bonding strength between the silicone rubber components of different types is not entirely satisfactory. Accordingly, the preparation of heterogeneous silicone rubber moldings can be extremely difficult depending on the size, shape, or placement of one of the silicone rubber elements.
A method for the manufacture of silicone rubber moldings which makes use of a semicured, silicone rubber particulate is disclosed in U.S. patent application Ser. No. 07/719,421, filed Jun. 24, 1991, and assigned to the same owner as this application.